1. Field of the Invention
This invention relates to a method of paste extrusion of lubricated polytetrafluoroethylene fine powder.
2. Description of the Related Art
Polytetrafluoroethylene (PTFE) fine powder is a type of PTFE that is made by aqueous dispersion polymerization, followed by coagulation of the dispersion and drying of the resultant coagulum to obtain the fine powder. Because the PTFE fine powder does not flow in the melt condition, the powder has been fabricated into an article by an extrusion method which does not require melt flow. This extrusion method is known as paste extrusion and is described for example in U.S. Pat. No. 2,685,707. In paste extrusion, a paste extrusion composition is formed by mixing PTFE fine powder with an organic lubricant which has a viscosity of at least 0.45 centipoise at 25.degree. C. and is liquid under the conditions of subsequent extrusion. The PTFE soaks up the lubricant, resulting in a dry, pressure coalescing paste extrusion composition that is also referred to as lubricated PTFE fine powder.
The process of paste extrusion may be understood with reference to FIG. 1 wherein there is shown an extrusion device 1 having a barrel section 3 and a die section 5. Inside device 1 there is defined a chamber 7 which terminates in an extrusion orifice 9. Lubricated PTFE fine powder, usually as a charge shaped under pressure to conform to chamber 7, is placed in chamber 7 and is then "paste extruded" by a ram 11 positioned in chamber 7 at the end opposite extrusion orifice 9 which moves toward extrusion orifice 9 to force the lubricated PTFE fine powder through extrusion orifice 9. Typically, there is a mandrel 13 positioned in chamber 7 so that the lubricated PTFE fine powder is paste extruded in the form of a sheet, rod, tubing or coating. The lubricated extrudate is also known as a lubricated green extrudate.
In FIG. 2 there is shown an extrusion device which has a mandrel with a configuration different from that in FIG. 1. Specifically, there is shown an extrusion device 21 having a barrel 23 defining a chamber 25. A ram 27 is positioned at one end of chamber 25. A mandrel 29 is fixed within chamber 25 and has a cylinder 31 attached at its distal end. Cylinder 31 has a conical surface and a cylindrical surface. As lubricated PTFE fine powder is paste extruded, the powder is pushed by ram 27 through chamber 25, flows around cylinder 31 and out of extrusion device 21 as extruded tube 32.
The lubricated green extrudate produced by paste extrusion may also be formed into an article of a desired shape. The paste extrusion is usually carried out at a temperature of 20 to 50.degree. C., though extrusion temperatures outside this range are known.
In most cases, the lubricated green extrudate is then heated, usually at a temperature of 100 to 250.degree. C., to make volatile and drive off the lubricant from the extrudate, followed by the optional step of sintering the PTFE. Some articles such as thread sealant tape are calendered and cut after drying, and are not subjected to sintering.
Lubricated PTFE fine powder includes PTFE primary particles which, during paste extrusion, are deformed into small interconnected fiber-like clusters, called fibrils. In the devices in both FIGS. 1 and 2, the lubricated PTFE fine powder is shaped into fibrils as it is forced around the mandrel and through the end of the extrusion device. In FIG. 1, this fibrillation occurs in the area between mandrel 13 and the walls of chamber 7 in die section 5. In FIG. 2, fibrillation occurs in the area between cylinder 31 and barrel 23.
These fibrils are oriented in the direction of the paste extrusion which means that after extrusion in devices such as are shown in FIGS. 1 and 2, the lubricated extrudate and articles formed therefrom are strong in the extrusion direction, but particularly weak in the extrusion-cross direction. For example, the fibrils formed by paste extrusion in device 21 in FIG. 2 are oriented in one direction, vertically, as the fibrils pass out of device 21. This weakness in the extrusion cross direction means that special care has to be taken to handle the lubricated extrudate before subsequent drying and optional sintering.
The anisotropy in properties of lubricated extrudate may be corrected after extrusion by stretching the extrudate or articles formed therefrom in two or more different directions, thereby reorienting the fibrils. Techniques of fibril reorientation by multi-lateral stretching of unsintered PTFE extrudate are known in the art such as are described in U.S. Pat. No. 5,321,109. Reorientation of the fibrils formed in the extrusion direction may also be accomplished by reorientation within the extrusion process. In each case, the prior art teaches forming the fibrils in the direction of the extrusion and then subsequently disorienting the fibrils by various methods, such as with expansion chambers, as shown in U.S. Pat. No. 3,315,020, or by applying a spiral-type movement to the lubricated extrudate, as shown in U.S. Pat. No. 3,008,187 and U.S. Pat. No. 4,8760,510. It is shown in the literature that the anisotropy in the unsintered lubricated green extrudate or articles made therefrom is generally extended to the sintered finished articles. This anisotropy may be compensated by subjecting the articles to long sintering cycles, but this is cumbersome and expensive. Increased isotropy achieved before sintering is usually also found in the sintered finished articles.
Further, even when the lubricated extrudate is subject to the prior art processes of stretching, the resultant articles are still difficult to handle and still have undesirable physical properties, including shrinkage in the extrusion direction.